The prime objective is to determine the duration and characteristics of naturally acquired immunity to M. pneumoniae infection among a cohort of 700 patients with prior M. pneumoniae infection as proven by isolation. Knowledge of natural immunity is essential to predicting the potential effectiveness and guiding the use of vaccines, which are now becoming available. The objectives include: 1) to determine the role of immunity (humoral, local or cell-mediated) following infection in prevention and/or enhancement of disease; 2) to evaluate the hypothesis that pneumonia occurs in persons over 5 years of age with M. pneumoniae infection following an earlier mild or asymptomatic sensitizing M. pneumoniae infection; 3) determination of which type of serum antibody(ies) most reliably indicate prior infections; 4) evaluation of the relation between type and titer of antibody and protection from M. pneumoniae infection; and 5) observing the duration of detectable antibodies following infection, the infection rates among approximately 250 school children and members of 40 families who have been followed longitudinally for several years with serological specimens will be continually followed. A new M. pneumoniae epidemic starting in the spring of 1974 will furnish an optimal and unique opportunity for determining the role of reinfection. The current epidemic will be compared with the one occurring in 1966-67 as to duration, peak rates, incidence by age and complications.